1. Field of the Invention
The present invention relates to a printing apparatus and, more particularly, to a printing apparatus in which a detection switch for setting an origin of a carriage is commonly used as a switch for starting automatic paper supply.
2. Description of the Prior Art
In an electronic typewriter, a printing operation for a printing paper held by a platen roller is executed while a carriage mounted with a printing mechanism moves along the platen roller. In an ordinary typewriter, a paper holding roller called a paper bail roller is provided for preventing the printing paper from floating up from the platen roller in printing operation. The paper bail roller can be moved to a pressed position, in which the paper bail roller is pressed against the platen roller, and to a released position, in which the paper bail roller is released from the platen roller, and in printing operation it is located at the pressed position. When the printing paper is inserted, the paper bail is located in the released position until the tip of the printing paper passes between the platen roller and the paper bail roller to make the insertion operation be performed securely.
There is well known technology in which the insertion operation of the printing paper is performed utilizing the operator's action of releasing the paper bail as a trigger; the technology is shown, for example, in a printing device disclosed in U.S. Pat. No. 4,498,795, in which attention is paid to the fact that the paper bail roller is released as mentioned in the above when the printing paper is inserted.
In an ordinary electronic typewriter, control for setting the origin position of a carriage is executed when power supply is switched on or when the cover of the printing mechanism is closed after a character wheel or the like is exchanged.
In an electronic typewriter in which paper supply operation is performed utilizing the releasing action of the paper bail roller as a trigger, a detection switch for setting the origin position of the carriage and another detection switch for detecting the operation of a paper bail arm for supporting the paper bail roller are separately provided to be used exclusively for each purpose. These detection switches are connected to different input ports of an input interface in a control device. The control device judges which detection switch the detection signal is from and, according to the detection signal, it executes origin setting control or automatic paper supply control.
In a conventional electronic typewriter in which automatic paper supply is possible, as mentioned in the above, a switch for setting the origin of the carriage and a switch for automatic paper supply are separately provided and these switches are connected to different input ports, so that there can be the following problems: the number of parts is increased; the same number of input ports as that of the switches is needed; and in some case an extra port chip is needed for only one port.